1. Field of the Invention
The invention relates to an alignment process and a display using the same. More particularly, the invention relates to a photo alignment process and a liquid crystal display using the same.
2. Description of Related Art
In a liquid crystal display, an electric field is applied to a liquid crystal layer through electrodes on two substrates. Liquid crystal molecules in the liquid crystal layer are polarized due to the electric field, such that the liquid crystal layer has a light transmittance corresponding to the electric field for displaying different grayscale images according to the value of the electric field. In order to provide stable boundaries for the liquid crystal molecules to align along a specific direction, an alignment layer is formed on a surface of at least one substrate contacting the liquid crystal layer. For the alignment layer to generate an alignment in a specific direction, conventionally, a rubbing process is performed to the alignment layer with a contact process. However, the alignment layer may be scratched or have particle contamination problem when this contact process is adopted. Consequently, a non-contact alignment process such as a photo alignment process is developed. In the photo alignment process, the alignment layer is irradiated with a linearly polarized light for alignment. A direction of an incident linearly polarized light determines an alignment direction of the alignment layer. An included angle of the incident linear polarized light and the alignment layer affects a pre-tilted angle when the liquid crystal molecules are aligned subsequently.
FIG. 1 is a schematic diagram illustrating a conventional alignment process. FIGS. 2A and 2B are enlarged schematic diagrams illustrating two regions shown in FIG. 1. Referring to FIG. 1, a linearly polarized light 120 with different direction irradiates the alignment layer 110 through a photomask 130 for making an alignment layer 110 have different alignment directions at different locations. Further, a substrate 140 or a light source is moved so that each region of the alignment layer 110 is irradiated by the linearly polarized light 120 to have a specific alignment direction. Generally, the photomask 130 is merely supported by a machine on the periphery so as to maintain a distance from the substrate 140. With an increase in dimensions of the substrate 140 and a demand for reducing process time, dimensions of the photomask 130 increase gradually, which may cause bent of the photomask 130 due to gravity. In addition, material of the photomask is flexible, so that distances from various locations of the photomask 130 to the alignment layer 110 are different. When incident angles of the linearly polarized light 120 are the same, a distance between an outlying area of the photomask 130 and the alignment layer 110 as shown in FIG. 2B is longer, and a distance between a center area of the photomask 130 and the alignment layer 110 as depicted in FIG. 2A is shorter. Accordingly, incident lights would generate slant incident angles relative to the substrate or the photomask respectively in photo alignment techniques. The slant incident angle is affected by the bent photomask, such that a projection of the slant incident light on the substrate has a displacement error. When the center area in the photomask 130 and the alignment layer 110 are precisely aligned, the outlying area in the photomask 130 and the alignment layer 110 have an alignment error therebetween, such that the alignment layer 110 can not be aligned satisfactorily.
FIG. 3A illustrates a relationship between a linearly polarized light and an alignment layer in a conventional photo alignment process. Referring to FIG. 3A, a slant incident angle between the linearly polarized light 120 and the normal vector of the alignment layer 110 is formed for generating a pre-tilted angle of the liquid crystal molecules induced by the alignment layer 110 in the conventional photo alignment technique. A wave vector 122 of the linearly polarized light 120, a polarization direction 124 of the linearly polarized light 120, and a normal vector 112 of the alignment layer 110 are co-planar. Under this condition, an alignment direction 114 of the alignment layer 110 is co-planar with the wave vector 122, the polarization direction 124, and the normal vector 112. The alignment direction of the alignment layer 110 is parallel to an orthographic projection of the polarization direction 124 on the alignment layer 110. Therefore, the wave vector 122 of the linearly polarized light 120, that is, the incident direction of the linearly polarized light 120, has to be adjusted in order to adjust the alignment direction 114 of the alignment layer 110. Hence, a relative position between a light source device (not shown) providing the linearly polarized light 120 and the substrate has to be adjusted many times for generating different alignment directions of the alignment layer 110, these adjustments increase the time cost of the photo alignment process and the occurrence of processing errors.
FIG. 3B is a schematic diagram showing the photomask adopted in the alignment process and the light incident direction in FIG. 1. Referring to FIG. 3B, the photomask 130 has a plurality of light transmissive regions 132 and the light transmissive regions are independent and not connected to one another. After passing the linearly polarized light through transmissive region 132 in a direction shown by an arrow, the linearly polarized light 120 ideally irradiates on a location framed by a dotted frame 152. However, when the photomask 130 bends so as to cause a distance between the photomask 130 and the substrate to change, a displacement error then results when the linearly polarized light 120 passes through the light transmissive region 132, and the linearly polarized light 120 irradiates a location framed by a dotted frame 154 instead of the dotted frame 152. The location of the dotted frame 154 includes offsets in both the X-axis and the Y axis comparing to the location of the dotted frame 152. When two adjacent regions in a pixel have different alignment directions, the displacement offsets resulting from the bending of the photomask 130 then cause the two adjacent regions with different alignment directions to overlap and thus affect the alignment. For example, the location of the dotted frame 154 is a region irradiated by the linearly polarized light 120 after passing through the light transmissive region 132. A location of the dotted frame 156 is a region irradiated by the linearly polarized light 120 after passing through the light transmissive region 132, where the light transmissive region 132 has moved to a bottom left region from current location due to the location offsets. As shown in FIG. 3B, the dotted frame 154 and the dotted frame 156 are partially overlapped such that the alignment direction of the overlapped part in the alignment layer is not definite. Under other conditions, a part of the alignment layer may not be irradiated by the linearly polarized light 120 and thus does not have a alignment direction.
Restricted by costs and technologies, the size of the photomask 130 does not equal to the size of the substrate 140 (denoted in FIG. 1). The photomask 130 has to be aligned with the substrate and be irradiated by the linearly polarized light 120 many times to complete the entire alignment process, resulting in high processing cost and low yield rate. To improve the above problems, a scanning alignment method has been developed. The light incident direction and the scanning direction are the same, thus the alignment direction which is parallel to the scanning direction is identical, which would reduce the projection offset effect caused by the bend of the photomask in the direction parallel to the scanning. However, the displacement error aforementioned still exists in this scanning method thus the scanning method is restricted to align the alignment layer in a direction parallel to the scanning direction. This method is currently used in wide viewing angle vertical photo alignment technology such as the mass production of inverse twisted nematic (ITN) products.
As for another wide viewing angle photo alignment technology, electrically controlled birefringence (ECB) has advantages such as faster liquid crystal responding speed and thus can have wide applications in liquid crystal displays. In a wide viewing angle ECB mode, the alignment directions of at least four directions are required in a pixel. Moreover, since the direction for aligning a top and a bottom substrates in the ECB mode have a 180° difference, irradiations from four different incident light directions are required respectively on the same sub-pixel for each substrate. Thus, the design of scanning alignment is not favorable in this case.